scholarly journals f(T) cosmology with nonzero curvature

2021 ◽  
Vol 36 (38) ◽  
Author(s):  
Andronikos Paliathanasis
Keyword(s):  
Cosmological Constant ◽  
Cosmological Solution ◽  
Constant Term ◽  
Analytic Solutions ◽  
De Sitter ◽  
Spatial Curvature ◽  
Field Equations ◽  
Background Space ◽  
Nonzero Curvature ◽  
Text Model

We investigate exact and analytic solutions in [Formula: see text] gravity within the context of a Friedmann–Lemaître–Robertson–Walker background space with nonzero spatial curvature. For the power-law theory [Formula: see text] we find that the field equations admit an exact solution with a linear scalar factor for negative and positive spatial curvature. That Milne-like solution is asymptotic behavior for the scale factor near the initial singularity for the model [Formula: see text]. The analytic solution for that specific theory is presented in terms of Painlevé series for [Formula: see text]. Moreover, from the value of the resonances of the Painlevé series we conclude that the Milne-like solution is always unstable while for large values of the independent parameter, the field equations provide an expanding universe with a de Sitter expansion of a positive cosmological constant. Finally, the presence of the cosmological term [Formula: see text] in the studied [Formula: see text] model plays no role in the general behavior of the cosmological solution and the universe immerge in a de Sitter expansion either when the cosmological constant term [Formula: see text] in the [Formula: see text] model vanishes.

scholarly journals Cosmological Bounce and Some Other Solutions in Exponential Gravity

Universe ◽  
2018 ◽  
Vol 4 (10) ◽  
pp. 105 ◽  
Author(s):  
Pritha Bari ◽  
Kaushik Bhattacharya ◽  
Saikat Chakraborty
Keyword(s):  
Cosmological Constant ◽  
Cosmological Solution ◽  
Constant Term ◽  
Gravity Theory ◽  
De Sitter ◽  
Sitter Solution ◽  
Higher Derivative ◽  
New Type ◽  
Theories Of Gravity ◽  
Theory Of Gravity

In this work, we present some cosmologically relevant solutions using the spatially flat Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime in metric f ( R ) gravity where the form of the gravitational Lagrangian is given by 1 α e α R . In the low curvature limit this theory reduces to ordinary Einstein-Hilbert Lagrangian together with a cosmological constant term. Precisely because of this cosmological constant term this theory of gravity is able to support nonsingular bouncing solutions in both matter and vacuum background. Since for this theory of gravity f ′ and f ″ is always positive, this is free of both ghost instability and tachyonic instability. Moreover, because of the existence of the cosmological constant term, this gravity theory also admits a de-Sitter solution. Lastly we hint towards the possibility of a new type of cosmological solution that is possible only in higher derivative theories of gravity like this one.

The influence of cosmological constant in temperature oscillations of a gas moving close to circular geodesic

2019 ◽  
Vol 34 (20) ◽  
pp. 1950109
Author(s):  
Leandro Cesar Mehret ◽  
Gilberto Medeiros Kremer
Keyword(s):  
Theoretical Model ◽  
Cosmological Constant ◽  
Constant Term ◽  
Circular Motion ◽  
Small Displacement ◽  
De Sitter ◽  
Normal Coordinates ◽  
Temperature Oscillations ◽  
Circular Geodesic ◽  
De Sitter Metric

The aim of this work is to analyze and to verify the effects of the charge and cosmological constant on the temperature oscillations that occur in a gas in a circular motion close to geodesic under the action of a Reissner–Nordström–de Sitter metric. The temperature oscillations are determined from Tolman’s law written in Fermi normal coordinates for a comoving observer. The temperature oscillations are calculated for a theoretical model obtained in the literature. Comparing the different configurations analyzed, it is possible to verify that the cosmological constant term causes a small displacement in the oscillation peaks. We also calculated the ratio between frequencies for some particular cases of the Reissner–Nordström–de Sitter metric and verified that the cases with null cosmological constant are closer of the 3/2 value found in QPOs. In another hand, the addition of the cosmological constant causes a direct increase of the ratio between frequencies.

scholarly journals Dimensionally Compactified Chern-Simon Theory in 5D as a Gravitation Theory in 4D

2017 ◽  
Vol 45 ◽  
pp. 1760005 ◽  
Author(s):  
Ivan Morales ◽  
Bruno Neves ◽  
Zui Oporto ◽  
Olivier Piguet
Keyword(s):  
Dimensional Space ◽  
Cosmological Solution ◽  
Space Time ◽  
Gravitation Theory ◽  
Simons Theory ◽  
De Sitter ◽  
Field Equations ◽  
Sitter Group ◽  
Dimensional Space Time ◽  
Chern Simons

We propose a gravitation theory in 4 dimensional space-time obtained by compacting to 4 dimensions the five dimensional topological Chern-Simons theory with the gauge group SO(1,5) or SO(2,4) – the de Sitter or anti-de Sitter group of 5-dimensional space-time. In the resulting theory, torsion, which is solution of the field equations as in any gravitation theory in the first order formalism, is not necessarily zero. However, a cosmological solution with zero torsion exists, which reproduces the Lambda-CDM cosmological solution of General Relativity. A realistic solution with spherical symmetry is also obtained.

GENERALIZED GRAVITY IN CLIFFORD SPACES, VACUUM ENERGY AND GRAND UNIFICATION

2011 ◽  
Vol 08 (06) ◽  
pp. 1239-1258 ◽  
Author(s):  
CARLOS CASTRO
Keyword(s):  
Cosmological Constant ◽  
Gauge Field ◽  
Constant Term ◽  
Gauge Field Theories ◽  
Field Equations ◽  
Spectral Action ◽  
Action Model ◽  
The One ◽  
Curvature And Torsion ◽  
Vacuum Solutions

Polyvector-valued gauge field theories in Clifford spaces are used to construct a novel Cl (3, 2) gauge theory of gravity that furnishes modified curvature and torsion tensors leading to important modifications of the standard gravitational action with a cosmological constant. Vacuum solutions exist which allow a cancelation of the contributions of a very large cosmological constant term and the extra terms present in the modified field equations. Generalized gravitational actions in Clifford-spaces are provided and some of their physical implications are discussed. It is shown how the 16 fermions and their masses in each family can be accommodated within a Cl (4) gauge field theory. In particular, the Higgs fields admit a natural Clifford-space interpretation that differs from the one in the Chamseddine–Connes spectral action model of non-commutative geometry. We finalize with a discussion on the relationship with the Pati–Salam color-flavor model group SU (4)C × SU (4)F and its symmetry breaking patterns. An Appendix is included with useful Clifford algebraic relations.

scholarly journals Comparative Analysis of Standard ΛCDM and ΛCS Models

2021 ◽  
Vol 57 (11) ◽  
pp. 1169
Author(s):  
V.E. Kuzmichev ◽  
V.V. Kuzmichev
Keyword(s):  
Cosmological Constant ◽  
Semiclassical Approximation ◽  
Cosmic Strings ◽  
Cosmological Parameters ◽  
Einstein Equations ◽  
Scale Transformation ◽  
De Sitter ◽  
Field Equations ◽  
Low Velocity ◽  
Sitter Model

We draw a comparison of time-dependent cosmological parameters calculated in the standard ΛCDM model with those of the model of a homogeneous and isotropic Universe with non-zero cosmological constant filled with a perfect gas of low-velocity cosmic strings (ΛCS model). It is shown that pressure-free matter can obtain the properties of a gas of low-velocity cosmic strings in the epoch, when the global geometry and the total amount of matter in the Universe as a whole obey an additional constraint. This constraint follows from the quantum geometrodynamical approach in the semiclassical approximation. In terms of general relativity, its effective contribution to the field equations can be linked to the time evolution of the equation of state of matter caused by the processes of redistribution of the energy between matter components. In the present article, the exact solutions of the Einstein equations for the ΛCS model are found. It is demonstrated that this model is equivalent to the open de Sitter model. After the scale transformation of the time variable of the ΛCS model, the standard ΛCDM and ΛCS models provide the equivalent descriptions of cosmological parameters as functions of time at equal values of the cosmological constant. The exception is the behavior of the deceleration parameter in the early Universe.

scholarly journals Null hypersurfaces in de Sitter and anti-de Sitter cosmologies

2018 ◽  
Vol 27 (04) ◽  
pp. 1850045
Author(s):  
P. A. Hogan
Keyword(s):  
Cosmological Constant ◽  
Gravitational Waves ◽  
Constant Curvature ◽  
Focus Attention ◽  
De Sitter ◽  
Einstein’S Field Equations ◽  
Field Equations ◽  
Null Hypersurfaces ◽  
Einstein's Field Equations ◽  
Line Elements

The study of gravitational waves in the presence of a cosmological constant has led to interesting forms of the de Sitter and anti-de Sitter line elements based on families of null hypersurfaces. The forms are interesting because they focus attention on the geometry of null hypersurfaces in spacetimes of constant curvature. Two examples are worked out in some detail. The first originated in the study of collisions of impulsive gravitational waves in which the post-collision spacetime is a solution of Einstein’s field equations with a cosmological constant, and the second originated in the generalization of plane fronted gravitational waves with parallel rays to include a cosmological constant.

scholarly journals Integrability and cosmological solutions in Einstein-æther-Weyl theory

2021 ◽  
Vol 81 (3) ◽  
Author(s):  
Andronikos Paliathanasis ◽  
Genly Leon
Keyword(s):  
Scalar Field ◽  
Cosmological Model ◽  
Conservation Law ◽  
Analytic Solutions ◽  
Special Interests ◽  
Field Equations ◽  
Background Space ◽  
Weyl Theory ◽  
Cosmological Solutions

AbstractWe consider a Lorentz violating scalar field cosmological model given by the modified Einstein-æther theory defined in Weyl integrable geometry. The existence of exact and analytic solutions is investigated for the case of a spatially flat Friedmann–Lemaître–Robertson–Walker background space. We show that the theory admits cosmological solutions of special interests. In addition, we prove that the cosmological field equations admit the Lewis invariant as a second conservation law, which indicates the integrability of the field equations.

scholarly journals Towards an explicit construction of de Sitter solutions in classical supergravity

2020 ◽  
Vol 2020 (10) ◽  
Author(s):  
Nakwoo Kim
Keyword(s):  
Boundary Conditions ◽  
Cosmological Constant ◽  
Explicit Construction ◽  
Analytic Solutions ◽  
De Sitter ◽  
Original Analysis

Abstract We revisit the stringy construction of four-dimensional de-Sitter solutions using orientifolds O8±, proposed by Córdova et al. (2019) [1]. While the original analysis of the supergravity equations is largely numerical, we obtain semi-analytic solutions by treating the curvature as a perturbative parameter. At each order we verify that the (permissive) boundary conditions at the orientifolds are satisfied. To illustrate the advantage of our result, we calculate the four-dimensional Newton constant as a function of the cosmological constant. We also discuss how the discontinuities at O8− can be accounted for in terms of corrections to the worldvolume action.

scholarly journals Instanton interaction in de Sitter space–time

2018 ◽  
Vol 33 (33) ◽  
pp. 1850200
Author(s):  
Dimitrios Metaxas
Keyword(s):  
Cosmological Constant ◽  
Decay Rate ◽  
De Sitter Space ◽  
Space Time ◽  
Tunneling Rate ◽  
Cosmological Horizon ◽  
De Sitter ◽  
Vacuum Decay ◽  
Background Space

Because of the presence of a cosmological horizon, the dilute instanton gas approximation used for the derivation of the Coleman–De Luccia tunneling rate in de Sitter space–time receives additional contributions due to the finite instanton separation. Here, I calculate the first corrections to the vacuum decay rate that arise from this effect and depend on the parameters of the theory and the cosmological constant of the background space–time.

scholarly journals Gravitational waves in Brans–Dicke theory with a cosmological constant

2021 ◽  
Vol 81 (4) ◽  
Author(s):  
Hatice Özer ◽  
Özgür Delice
Keyword(s):  
Cosmological Constant ◽  
Gravitational Wave ◽  
Weak Field ◽  
Critical Distance ◽  
Massless Scalar ◽  
De Sitter ◽  
Field Equations ◽  
Wave Solutions ◽  
Free Particles ◽  
The Waves

AbstractWeak field gravitational wave solutions are investigated in Brans–Dicke (BD) theory in the presence of a cosmological constant. In this setting the background geometry is not flat but asymptotically de-Sitter. We investigate the linearised field equations, and their gravitational wave solutions in a certain gauge choice. We will show that this theory leads to massless scalar waves as in original BD theory and in contrast to massive BD theory. The effects of these waves on free particles and their polarization properties are studied extensively and effects of the cosmological constant is analyzed in these phenomena in detail. The energy flux of these waves are also discussed in this background. By analyzing this flux, we obtain a critical distance where the waves cannot propagate further, which extends Cosmic no Hair Conjecture (CNC) to BD theory with a cosmological constant.

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